The combination of GC and FTIR Analytical Techniques is well known whereby a sample is separated into its constituents in a GC. An FTIR Spectroscope and data system detects and analyzes the constituents in a rapid and highly sensitive manner. GC/FTIR systems generally comprise a GC including an oven containing a separating column. A carrier gas continuously flows through the column and a sample, generally in liquid form, is injected into the GC and vaporized so that its constituents are picked up by the carrier gas and separated, by partitioning, as they flow through the column. The effluent from the column is passed through a light pipe whereby the various constituents or peaks thereof flow at different times through the light pipe. Infrared energy from an interferometer is directed through the light pipe and is absorbed by the constituents in a manner indicative of the quantity and type of constituent. The IR energy is detected so as to create an electrical signal or interferogram which is then digitized and inputed into a data processing system. The interferogram is then subjected to a Fourier transform and other mathamatical computations to provide the desired analysis and output.
In prior art GC/FTIR systems, the separating column is located within an oven and the light pipe is located external to the oven. A transfer tube is connected to the exit end of the separating column and to the input end of the light pipe. Various fittings are used at the different connections and it becomes a difficult problem to achieve a laminar gas flow from a column, through the transfer tube and the light pipe. Some of the fittings may introduce turbulence and eddy currents and possibly be so shaped as to create dead volumes. The net result is that peak broadening and tailing might occur that affect the resolution of the system. This problem becomes more acute when the column is a capillary column having a relatively long length in relationship to its inner diameter. The capillary column inner diameter is generally small and it might differ from the inner diameter of the transfer tube and light pipe so that the problem further occurs of maintaining the constituents in a separated or resolved state.
In order to produce spectra wherein spectral band differences are well defined, a high signal-to-noise ratio is desired. Such results could be obtained by having a highly concentrated sample component in the IR light pipe or IR sample beam. Heretofore, this was accomplished by using a relatively wide bore GC column, which could contain a large amount of sample in the column and therefore provide the higher concentration to achieve the high signal-to-noise ratio. However, wide bore columns generally provided low resolution and poor separation efficiency.